The ''Xenodiscidae'' are the earliest of the

Ceratitida Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from t ...

and comprise Middle and Upper

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Pale ...

genera characterized by compressed, discoidal, evolute shells with rounded to acute venters and commonly with lateral ribs. Sutures are goniatitic to weakly ceratitic.Arkell et al, 1957 The Xenodiscidae, which are part of the superfamily Xenodiscoidea, are derived from the

Daraelitidae The Daraelitidae form a family in the ammonoid order Prolecanitida from the Upper Mississippian - Middle Permian characterized by discoidal shells with no prominent sculpture, moderately large umbilicus, and goniatitic or ceratitic sutures with ...

, a family in the

Prolecanitida Prolecanitida is an order of extinct ammonoid cephalopods with discoidal to thinly lenticular shells with goniatitic or ceratitic sutures and which retained the simple retrochoanitic siphuncle with backward extending septal necks. As typical for ...

(ibid). In turn, the Xenodiscidae provided the root stock for the subsequent expansion and diversification of the Ceratitida in the Triassic.

Notes

References

* Arkell et al., 1957; Mesozoic Ammonoidea,

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

, Part-L, Ammonoidea; Geological Soc. of America, reprinted 1990.
The Paleobiology Database
July 2009 Xenodiscoidea Ceratitida families Guadalupian first appearances Lopingian extinctions {{Ceratitida-stub